Electroless, i.e., autocatalytic, metal deposition solutions for the formation of metal layers on non-metallic or metallic substrates are well known in the art. These are characterized by the capacity to deposit metal in virtually any desired thickness on a wide variety of surfaces without the need for an external supply of electrons. Such solutions differ from electroplating baths which require an externally supplied source of electrons, and they also differ from displacement metal plating and metal mirroring methods where the metal deposited is only a few millionths of an inch in thickness. Electroless metal deposition solutions are especially suitable for forming metal layers on the surface of non-metallic or resinous articles which have been pretreated to render the surface catalytic to the electroless reception of metal.
Special mention is made of the use of electroless metallizing procedures in the plating of plastics generally and the manufacture of printed circuit boards particularly. In the plating of plastics, a thin layer of copper is electrolessly deposited on the sensitized surface of a resinous article, e.g., an insulating material, to produce a metallized or metal plastic part for use, e.g., in the automobile industry, as grills, door knobs and the like. In the manufacture of printed circuit boards, a thin layer of copper is electrolessly deposited on a sensitized surface of an insulating substratum, selected areas of the surface of the electroless deposit are masked, the initial layer of unmasked copper is then built up by electroplating, and the masked areas of copper are etched away after removal of the masking layer to leave the desired conducting pattern of copper on the surface. In another procedure, selected areas of the surface of the insulating substratum are sensitized in the form of a desired printed circuit pattern and copper is electrolessly deposited on the sensitized areas to form the desired circuit pattern. In the manufacture of printed circuit boards, electroless metal deposition techniques are also often used to plate the sensitized walls of through-holes formed in the insulating article in order to e.g., produce electrically conductive connections, so-called plated through holes, between circuit patterns formed on opposite sides of the article surface.
A shortcoming of early processes for the electroless deposition of copper was that the deposition solution was unstable initially or became unstable after a relatively brief operating period and then had to be dumped. Such solutions also tended to produce electrolessly formed copper deposits which were dark in color and which tended to flake off the substratum on which deposition was taking place. To overcome such shortcomings, the art has proposed a number of compounds as stabilizing agents for prolonging the useful life of electroless metal deposition solutions and for improving the quality of copper deposit. These include 2-mercaptobenzothiazole, in Pearlstein, U.S. Pat. No. 3,222,195; 2,5-dimercapto-1,3,4-thiodiazole and 8-mercaptopurine, in Jackson, U.S. Pat. No. 3,436,233; o-phenanthroline, in Stone, U.S. Pat. No. 3,615,735; 1-phenyl-5-mercaptotetrazole, in Jonker et al, U.S. Pat. No. 3,804,638; 2,2'-dipyridyl and 2-(2-pyridyl)-benzimidazole, in Hirohata et al, U.S. Pat. No. 4,002,786; and benzothiazole-thioetherpolyethyleneglycol, in Molenaac et al, U.S. Pat. No. 3,843,373.
Still other stabilizing agents are disclosed in Schneble et al, U.S. Pat. No. 3,257,215, for example, thiazoles isothiazoles and thiozines, Maguire, U.S. Pat. No. 3,793,038, for example, benzotriazole, diazole, imidazole, guanidine, pyrimidine, and others and Torigai et al, U.S. Pat. No. 3,377,174, for example, 2,2'-biquinoline, 2,9-dimethylphenanthroline and 4,7-diphenyl-1,10-phenanthroline.
Schoenberg, U.S. Pat. No. 3,708,329, discloses that the addition of a heterocyclic aromatic nitrogen compound having up to 3 rings with a hydroxy group bonded to one of the rings, results in a marked increase in the stability of electroless copper plating baths without adversely affecting the plating rate. See also Schoenberg, the Journal of the Electrochemical Society, 118, 1571 (1971). Although Schoenberg in U.S. Pat. No. 3,708,320 talks about improving plating rate, the fastest bath described by Schoenberg has a room temperature plating rate of only 3.1 microns per hour. Even that slow rate, however, is higher than any long term rate mentioned in any of the other prior art references identified above. The fastest reported long term rate for electroless copper plating solutions currently available commercially, e.g., Dynachem DC-920 and MacDermid 9027, is 5 microns per hour. U.S. Pat. No. 3,377,174 reports a short term plating rate of 0.5 microns in a five minute period.
Heretofore, it was considered necessary to operate electroless copper solutions at a low rate, i.e., less than about 6 microns per hour so as to produce a copper deposit of good quality, i.e., a coherent, structurally stable, thin film of copper adherent to the surface being coated. The experience in the art has further been that plating rates above about 6 microns per hour resulted in the production of a copper deposit of poor quality, i.e., one which flakes off or tends to flake off the surface or which was non-coherent.
As used herein, the phrase adherent copper deposit refers to an electrolessly formed copper deposit which can be stripped from a plated insulating substratum in the form of a thin, integral film such that when stripped, retains its structural integrity or cohesiveness as a film without crumbling.
As used herein, the phrase non-adherent copper deposit refers to an electrolessly formed copper deposit which flakes or tends to flake off the coated substratum. Such a deposit lacks cohesiveness and cannot be stripped from the insulating substratum in the form of a thin, stable, structurally integral film.
It is one object of this invention to increase the rate at which copper can be electrolessly formed.
It is a further object of this invention to provide procedures and compositions for increasing the rate for electrolesssly forming an adherent copper deposit.
Another object of this invention is to provide electroless copper deposition solutions having high plating rates.
Still another object of this invention is to provide compositions and procedures for electrolessly forming adherent copper deposits at high rates heretofore considered unachieveable.
Other and further objects of this invention will be clear from the description which follows and from the examples.
In accordance with the invention, it has been found that these and other objects may be achieved by operating a given electroless copper solution of the type disclosed in the presence of an acceleratoor or depolarizing agent at a pH greater than the peak plating rate pH of the solution without such an agent. In general, the depolarizing agent should be capable of achieving at least 20% and up to 100% or between about 35% and 90% depolarization of the anodic partial reaction or the cathodic partial reaction of the solution, or both. Stated differently, the depolarizing agent should be capable of accelerating by at least 20% and up to 100% or between about 35 and 90%, the cathodic partial reaction or the anodic partial reaction of the solution, or both.
The increase in the rate at which adherent copper may be deposited from a given electroless copper solution by practice of this invention will vary over a wide range depending upon the formulation used and the quality of copper desired. In general, rate increases achieved by practice of this invention will be at least up to 300% or more depending upon solution formulation. However, rate increases of up to 1 or 11/2 orders of magnitude, i.e., 10 times (1000%) or even 50 times (5000%) are possible. Achievement of such rate increases was unexpected and surprising.
Similarly, the rate at which adherent copper may be deposited for a prolonged period of time from a given electroless copper solution by practice of this invention will vary over a wide range, again depending upon the formulation used and the quality of copper desired. With additive present, the solutions covered herein are characterized by a room temperature plating rate above 7 microns per hour, and generally above 9 microns per hour, or between about 9 and 25 microns per hour and higher and are characterized by the ability to electrolessly form copper at a rate of up to at least 30 microns per hour for a period of at least 15 minutes. Elevated temperature rates of up to 70 microns per hour or even higher are however possible. Here again, achievement of such rates was unexpected and surprising. Moreover, such rates may be achieved for periods of time ranging from one or several minutes up to prolonged periods up to eight hours or more. Typical are operating times of about 5 minutes to about 8 hours. With proper replenishment, the solutions may continue in use for extended periods of time, e.g., weeks. It should be noted that the fast rates of the solutions generally make prolonged plating periods unnecessary.
Electroless formation of copper in accordance with this invention will result in many operating advantages, including shorter plating times and, concomitantly, increased production capacity. Compared to commercial practices now available, the procedures and compositions of this invention require less equipment, lower capital investment costs and lower energy requirements. Unlike the current commercial practices, the procedures herein taught are particularly suitable for use in automatic plating systems with relatively short dwell times.